The present invention relates to a boom of a bucket type excavator such as a hydraulic shovel and a method for making such boom.
As shown in FIG. 1, in a hydraulic shovel of a bucket type excavator, an upper vehicle body 2 is turnably mounted on a lower running body 1, a boom 3 is vertically swingably mounted to the upper vehicle body 2, an arm 4 is vertically oscillatably mounted to the boom 3, and a bucket 5 is vertically oscillatably mounted to a tip end of the arm 4. A boom cylinder 6 is connected between the upper vehicle body 2 and the boom 3, an arm cylinder 7 is connected between the boom 3 and the arm 4, and a bucket cylinder 8 is connected between the arm 4 and the bucket 5.
The hydraulic shovel vertically swings the boom 3, the arm 4 and vertically oscillates the bucket 5, and at the same time, laterally turns the upper vehicle body 2, for carrying out operations such as excavation and loading to a dump truck.
As shown in FIG. 2, the boom 3 comprises a boom body 10 of boomerang shape as viewed from side, a vehicle body-mounting bracket 11 connected to one longitudinal end of the boom body 10, and an arm-connection bracket 12 connected to the longitudinally other end of the boom body 10. As shown in FIG. 3, the boom 10 is formed into a hollow structure of rectangular cross section in which an upper lateral plate 13, a lower lateral plate 14, and left and right vertical plates 15 and 15 are welded at right angles to one another so as to reduce the boom body 10 in weight.
At the time of excavation, the boom 3 is driven in the vertical direction for inserting the bucket into earth and sand, a vertical load F1 is applied to the boom 3 as shown in FIG. 1. When the excavator turns around the upper vehicle body 2 for loading the dipped up earth and sand onto a dump truck or the like, a lateral load F2, and a torsion load F3 are applied to the boom 3. Therefore, the boom 3 is formed such that the boom 3 can withstand the loads and is not deformed. For example, against the vertical load F1, a height H is increased as compared with a width W as shown in FIG. 3. Against the lateral load F2 and the torsion load F3, a partition wall 16 is connected such that an opened box-like structure is formed as shown in FIG. 3, and a vertical plate of a boom cylinder boss 18 is provided with a cross section restraint material such as a pipe 17 (FIG. 4) for dispersing the torsion force and load.
In the hydraulic shovel, a counter weight 9 is provided at a rear portion of the upper vehicle body 2 in accordance with the excavation ability of a working machine comprising the upper vehicle body 2 which is a main portion, the boom 3, the arm 4 and the bucket 5. If the working machine is reduced in weight, the weight of the counter weight 9 can be reduced, the rearward projecting amount of the upper vehicle body 2 can be reduced and therefore, a turning radius of the rear end of the upper vehicle body 2 can be reduced.
If the working machine comprising the boom 3, the arm 4 and the bucket 5 is reduced in weight, it is possible to increase the volume of the bucket correspondingly and thus to increase the working load capacity.
Further, the boom 3 is vertically swung by the boom cylinder 6, and a portion of a thrust of the boom cylinder 6 supports the weight of the boom 3. Therefore, if the boom 3 is reduced in weight, the thrust of the boom cylinder 6 effectively can be utilized as the vertical swinging force of the boom 3.
In general, when considering a strength of the working machine of the bucket type excavator, as the simplest method, a working machine is replaced with a beam or a thin pipe which is discussed in material mechanics and a strength with respect to the bending and torsion can be evaluated.
That is, bending stress s, and shearing stress t generating on a cross section can be obtained by the following general formulas (1) and (2):
s=M/Zxe2x80x83xe2x80x83(1)
(wherein, s: bending stress generating on a cross section, M: bending moment applied to the cross section, Z: cross section coefficient)
t=T/2Atxe2x80x83xe2x80x83(2)
(wherein, t: shearing stress, T: torsion torque, A: projection area of neutral line of cross section plate thickness, t: thickness of cross section plate)
An appropriate shape of the cross section can be determined from the results of the above calculation and permissible stress of the material to be used. Similarly, deflection of the beam and torsion of the axis can be calculated using general formula of the material mechanics, and such calculation, rigidity of the working machine can also be evaluated.
However, if a working machine designed in accordance with the above evaluation method is actually produced and a stress test is carried out, the result of the test is different from a stress value calculated during the evaluation in many cases. For this reason, in recent years, simulation by a computer using finite element method (FEM) is employed as the evaluation method for enhancing the precision of the stress evaluation. If the stress is calculated using the FEM simulation, it can be found that a cross section of a working machine which was considered as beam and axis of material mechanics is changed in shape before and after the load is applied. From this fact, it can be understood that a stress calculated using the general formulas of the material mechanics derived based on a presumption that a shape of a cross section is not changed and a stress measured when a stress test is actually carried out do not coincide with each other.
In the case of a conventionally used working machine having a rectangular cross section, there are two factors for determining a deformation strength of the cross section, i.e., rigidity of a rectangular angle portion and rigidity of a rectangular side portion in the outward direction of a surface. When each of the two rigidity does not have sufficient strength against a load, the cross section is deformed as shown in FIG. 5, and an excessive load is applied to the rectangular angle portion. To prevent those, a cross section restraint material such as a partition wall is required for a portion in which its cross section is deformed, but if such material is provided, productivity of the working machine is lowered.
If the above facts are applied to the boom 3, the boom 3 is of hollow shape of rectangular cross section as shown in FIG. 3, rigidity of the cross section is determined by bending rigidity of an angle portion a, bending rigidity (rigidity in the outward direction of surfaces) of the four surfaces (the upper lateral plate 13, the lower lateral plate 14, and the left and right vertical plates 15 and 15). That is, influence of the bending rigidity of the surfaces and the bending rigidity of the angle portion is great with respect to the deformation of the cross section. For example, in FIG. 3, when the lower plate 14 is fixed, and a load F shown with the arrow F is applied, as shown in FIG. 5 schematically, each of the angle portions a is bent and deformed, the upper plate 13 and the left and right vertical plates 15 and 15 are bent and deformed in the outward direction of the surfaces (thickness direction). When the thickness of the plate is reduced, reduction of rigidity in the outward direction of the surface is proportional to the third power of a ratio of reduction of the plate thickness.
For these reasons, if the thickness of each plate is reduced to increase the cross section, when the lateral load F2 and the torsion load F3 are applied to the boom 3, a deformation is generated in the light weight boom 3 as shown with the arrows b and c in FIG. 3, and the rigidity of the entire boom is largely lowered. Therefore, the above-described cross section restraint material such as the partition wall 16 and the pipe 17 must be reinforced, the weight of the boom is increased because of the reinforced cross section restraint material, the structure is complicated because of the partition wall 16 and the pipe 17, and there is a problem with the productivity due to increase in welding requirements.
Further, as shown in FIG. 2, the boom 3 is provided with a boom cylinder boss 18 for connecting the boom cylinder 6, and an arm cylinder bracket 19 for connecting the arm cylinder 7. If the thickness of each of portions to which the boss 18 and the bracket 19 are to be connected, e.g., the left and right vertical plates 15, 15 and the upper lateral plate 13 is reduced, the rigidity in the outward direction of the surface is lowered. Therefore, in some cases, this further increases the deformation in the outward direction of the surface and reduces the rigidity of the boom 3, and a deformation shown with a phantom line in FIG. 3 is generated. Thus, it is difficult to reduce the thickness of plate material forming the boom body 10.
Further, since the plate members forming the boom body 10 are welded to one another at right angles, if the thickness of the plate members is reduced, the weld jointing efficient is lowered, and it is difficult to secure the durability of the angle joint and thus, it is difficult to reduce the thickness of the plate members forming the boom body 10.
Furthermore, in the case of the conventional boom, the upper lateral plate 13, the lower lateral plate 14 and the left and right vertical plates 15, 15 are formed by cutting them in accordance with the shape of the boom body 10, and the vehicle body-mounting bracket 11 and the arm-connection bracket 12 are welded to the boom body 10. Therefore, working of each of the plate members is complicated, the welding portion (welding line) is long, many steps are required to produce the boom and thus, the producing method is complicated.
A boom shown in FIG. 6 in which one sheet of plate is bent into U-shape and the upper lateral plate 13 and the left and right vertical plates 15, 15 are formed into one unit is known. However, in this case also, a step for cutting the plate and the lower lateral plate 14, a step for bending, and a step for welding two welding portions (welding lines) are required and thus, many steps are required and this method is complicated.
Therefor, it is an object of the present invention to provide a boom of a bucket type excavator and a method of making same which can solve the above problems.
In a boom of a bucket type excavator of a first embodiment of the invention having a boomerang-like shape in which a base end of the boom is mounted to a vehicle body and an arm is mounted to a tip end of the boom, a boom body is hollow and triangular in cross section.
According to the first embodiment, since the boom body 23 is triangular in cross section, due to characteristics of a triangle that its cross section is less prone to be deformed in the outward direction of surface by load, the boom body 23 can keep its cross section shape and secure rigidity therein without using a cross section restraint material such as a pipe. Therefore, the plate thickness of the boom body 23 can be reduced to reduce its weight, and the cross section restraint material such as a partition wall and the pipe is unnecessary and thus, its structure is simple, and the number of portions requiring welding is small and therefore, durability and productivity are enhanced. Therefore, the weight of the boom can be reduced, and excellent durability and productivity achieved.
In a boom second embodiment, the boom body has a cross section of the first embodiment in which three sides are straight, and each of connected portions of the two sides is of arcuate shape.
According to the second embodiment, since the cross section of the boom body 23 in which the three sides are straight, and each of connected portions of the two sides is of arcuate shape, the sectional area can be increased such that it inscribes a sectional area of a conventional boom, the cross section performance can be maintained, and since the angle portion is arcuate in shape, stress can be dispersed. Therefore, a large sectional area can be secured, the cross section performance can be maintained, and the rigidity of the boom is enhanced.
In a boom of a third embodiment, the boom body 23 has a triangle cross section of the second embodiment in which a lower surface thereof is a triangular base side, and an upper surface thereof is an apex of the triangle.
When the boom is curved downward into a boomerang shape and a vertical size of its intermediate portion is greater than those of opposite ends, the boom has properties that if a lateral load (F2 in FIG. 1) or a torsion load (F3 in FIG. 1) is applied to a tip end of the boom, length of a force transmitting path of the upper surface side is longer than that of the lower surface side and therefore, there is a tendency that a burden of a load of the lower surface side which is shorter in length is greater. Therefore, as in a third embodiment form, if the lower surface is formed into a base of a triangle, the cross section performance can be exhibited more efficiently as compared with a structure which is turned upside down, and the weight can further be reduced. When the weight reduction is taken into consideration, it is advantageous that the base is disposed at the shorter lower surface side as compared with a case in which the base having great weight is disposed at the longer upper surface side.
In a boom of a fourth embodiment, an arm cylinder bracket 26 is jointed to an upper surface of the arc connected portion of the two sides, and since the top of the boom body 23 has great rigidity, even if the plate thickness of the mounting portion of the arm cylinder bracket 26 is thin, the boom is not deformed. With this structure, the plate thickness of the mounting portion of the arm cylinder bracket 26 of the boom body 23 can be thin to further reduce the weight of the boom.
In a boom of a fifth embodiment, the boom body 23 has a substantially triangular cross section of the second embodiment in which a lower surface thereof is a triangular base side, an upper surface thereof an apex of the triangle, the top comprises two arcuate portions and a flat portion, and an arm cylinder bracket 26 is jointed to the flat portion of the top.
According to the fifth embodiment, since the top of the boom body 23 is a flat portion, when the arm cylinder bracket 26 is welded to the flat top, edge preparation of the arm cylinder bracket 26 is unnecessary and the throat depth of the weld joint can be secured by using a fillet weld joint. Therefore, the welding operation of the arm cylinder bracket 26 to the top of the boom body 23 is facilitated, and even if the plate thickness is thin, welding strength can be maintained.
In a sixth embodiment, and any one of the fourth and fifth embodiments, the boom body 23 is provided at its central portion with a pin fitting hole 45 for mounting a boom cylinder, an arm-connection bracket 24 is jointed to a tip end of the boom body 23, and a vehicle body-mounting bracket 25 is jointed to a base end of the boom body 23.
Since the boom body 23 is provided with the pin fitting hole 45, and the arm-connection bracket 24 and the vehicle body-mounting bracket 25 are welded to the boom body 23, the number of welding lines and constituent ports are small. Therefore, weight can further be reduced, and since the constituent parts is few, labor of management can be omitted. Further, when a vertical load (F1 in FIG. 1) is applied to such a boom, a portion of the boom body 23 which is closer to the front end than the pin fitting hole 45 receives a burden of load at its lower surface, and a portion of the boom body 23 which is closer to the vehicle body than the pin fitting hole 45 receives the burden of load at its upper surface side, but the tensile load on the front lower surface side, and the compressing load on the vehicle body side upper surface side are great. In terms of strength, since the tensile load is greater than the tensile load, if the cross section shape of the boom body 23 is formed such that its lower surface becomes a base side, it is advantageous with respect to deformation. It is necessary to guard against surface buckling for a portion where the compressing load is great (vehicle body side upper surface side), and it is advantageous against deformation such as surface buckling by disposing the top of the triangle on the above-described portion rather than disposing the base surface on this portion.
In a seventh embodiment, one longitudinal end of one boom front member 20 which is hollow and triangular in cross section and one longitudinal end of a boom rear member 21 which is hollow and triangular in cross section are connected to a boom intermediate member 22 having a pin fitting hole 45 with the same cross section shape as each of the cross sections, thereby forming the boom body 23, the arm-connection bracket 24 is jointed to the longitudinal other end of the boom front member 20, and the vehicle body-mounting bracket 25 is jointed to the longitudinal other end of the boom rear member 21.
Since the boom body 23 comprises the boom front member 20, the boom intermediate member 22 and the boom rear member 21, the handling is facilitated and large-scaled production facilities are unnecessary. That is, by dividing the boom body into the three elements, i.e., the boom front member 20, the boom intermediate member 22 and the boom rear member 21, the large-scaled production facilities are unnecessary and the handling is further facilitated.
A method for making a boom of a bucket type excavator according to the invention comprises the steps of: bending substantially rectangular plate material 62 having two long sides 60, 60 and two short sides 61, 61, thereby forming a hollow member which is triangular in cross section, and welding butted portions of the two long sides 60, 60, thereby forming a boom body 23.
Since one sheet of plate material is bent and the butted portions are welded to form the boom body 23, the working of the plate material is easy, and the welding portions (welding line) is short. With this method, the steps of making the boom body 23 are easy, and the boom can be produced with facility.
Further, according to the invention, the boom body 23 can have a cross section in which three sides are straight, and each of connected portions of the two sides is of arc shape, the boom body 23 has a triangle cross section in which a lower surface thereof is a triangular base side, an upper surface thereof is a tip of the triangle, and butt-welded portions of the two long sides are disposed on the lower surface. Because the welding portion is disposed on the lower surface, outward appearance can be enhanced as an added advantage of the invention.